We still have a (big) safety problem

Suppose we have

```haskell
x :: Either Int Char
x = Right 'a'

y :: Either Bool Char
y = Right 'a'
```

Thanks to some CMM-level optimizations, it is possible that `x` and `y` will live at the same address. I can insert `(x, Identity (Left 3))` into the map, then perform a lookup with `y` and get something utterly wrong.

Start by reproducing the issue's example with x :: Either Int Char, y :: Either Bool Char, and the map insertion of (x, Identity (Left 3)). Investigate the lookup path under CMM-level optimizations; done means a lookup with y cannot return the value associated with x when both share an address.

Written by the indexing model from the issue text.